1. Field of the Invention
The invention relates to an implantable sensor for amperometric measurements in body liquids, such as blood or tissue liquid, with a measurement electrode, preferably provided by a platinum or gold wire, and with a reference electrode tubularly surrounding the measurement electrode.
2. Brief Description of the Background of the Invention Including Prior Art
Such sensors are of interest in particular in connection with the control of blood sugar and tissue sugar for diabetics and they are suitable to be employed in a portable diagnostic and/or therapeutic apparatus.
A patient, suffering from the metabolic disease diabetes mellitus, has to balance several times daily his or her blood-sugar level by injection of insulin. In order to be protected against substantial metabolic deviations, in addition, blood-sugar controls are required. The blood-sugar controls require each time a pricking of the finger tip, which represents a substantial burden, in particular for small children and adolescents. On the other hand, the continuously changing metabolic situation of a diabetic person can result in the feared diabetic delayed lesion and late damages, which are caused in particular based on changes of the vessels and a worsening of the oxygen supply to the individual organs.
In order to alleviate the control of blood sugar, electrochemical enzyme sensors have been employed successfully for some time already for determination of blood glucose in vitro. The respective apparatus are however still fairly expensive and heavy in their construction such that they can only be considered for a stationary treatment.
The use of implantable enzyme sensors has already been proposed, where the implantable enzyme sensors are to be employed in connection with portable measurement and evaluation apparatus. A conventional needle-shaped glucose sensor of this kind exhibits a measurement anode consisting of a platinum wire, a reference cathode consisting of silver, and a glass insulator disposed between the measurement anode and the reference cathode, an enzyme layer of an immobilized glucose oxidase disposed on the active electrode surface, as well as a porous membrane of polyurethane covering the enzyme layer. The limiting diffusion currents, measured at a polarization voltage of about 600 millivolts, are from several 100 microamperes to several nanoamperes, depending linearly on the glucose concentration in the presence of .beta.-D glucose in an electrolytic body liquid. However, in case of in vivo measurements, there eventually occurs a drifting to lower limiting diffusion currents over the course of time which is presumably caused by deposits of fibrin from the tissue and/or blood. This drifting can be compensated in fact by recalibration over certain periods of time. If these recalibrations cannot be performed automatically then, however, in this context, manipulating difficulties occur, which could oppose a wide application of such sensors. This holds the more, since the reference electrode, made of silver and therefore determining the cost, still requires material costs that are too high for such a consumer article. Finally, the data-capturing and data-evaluating apparatus are missing for a practical use of the implantable glucose sensors, which apparatus is sufficiently compact for transportation at or in the body and which are suitable for a long-term battery operation.